It's a denarius of the roman emperor Vespasian. If you look around the edge from the top of the head, you'll see VESPASIAN. Looks like the reverse is the god Jupiter, like in the picture below. The reverse legend says 'IOVIS CVSTOS' - a plea for Jupiter to keep Rome safe.

EDIT: was a little silly and put an example that is actually Titus, who issued a very similar coin - I think this one is Vespasian. The difference is in the legend at the bottom of the bust - it will read T CAESAR IMP VESP... if it it Titus, and IMP CAESAR VESP... if it is Vespasian, like yours. I have replaced the picture with one actually from Vespasian.

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